Saponins are a group of sapogenin glycosides obtainable from many plants. Each saponin consists of a sapogenin which constitutes the aglucon moiety of the molecule and a sugar. The sapogenin may be a steroid or a tri-terpene and the sugar moiety may be glucose, galactose, a pentose or a methylpentose. Aqueous solutions of saponins foam like soap on shaking and are used, inter alia, as detergent.
Saponins were used in very early fire fighting foams as foaming agents but have long since been discontinued due to their high cost, which makes them unsuitable for use as foaming agents, given the existence of cheaper alternatives.
Synthetic fire fighting foam formulations, such as aqueous film forming foams (of either the alcohol resistant type or non-alcohol resistant type), high expansion foams and class A foams (which includes forest fire foams) use a synthetic hydrocarbon surfactant or detergent as a foaming agent. Aqueous film forming foams (AFFF's), in particular, consist of both synthetic hydrocarbon surfactant foaming agents and a fluorocarbon surfactant added to lower the surface tension.
The basic concept behind AFFF formulations is that it is possible to float a film of water on top of a lower density hydrocarbon fuel if the surface tension is sufficiently low and the interfacial tension between the two liquids is in the appropriate range. The fluorocarbon surfactant is able to achieve this low surface tension.
Aqueous film forming foams also use a non-ionic hydrocarbon surfactant, which is usually of the ethoxylated octyl phenol type (such as TRITON X 102), in combination with the fluorocarbon surfactant to lower the level of fluorocarbon surfactant required. Without the addition of the non-ionic hydrocarbon surfactant, higher levels of the relatively expensive fluorocarbon surfactant is required. Alkyl polyglycoside surfactants can also be used to replace the ethoxylated octyl phenol type surfactant.
A typical 6% AFFF formulation of the prior art has the following general composition:
______________________________________ COMPONENT RANGE ______________________________________ Fluorocarbon Surfactant 0.5% to 2% Amphoteric Hydrocarbon Surfactant 1% to 5.5% Anionic Hydrocarbon Surfactant 0% to 2.5% Non Ionic Hydrocarbon Surfactant 0.5% to 2% Glycol/Glycol Ether 2% to 10% Buffer 0% to 2% Sequestrant 0% to 1% Corrosion Inhibitor 0% to 2% Water Balance ______________________________________
A limitation of hydrocarbon surfactants resides in the heat resistance of the foam produced. Furthermore, fluorocarbon surfactants are very expensive and not particularly environmentally friendly.
We have found that the use of low levels of a saponin can significantly improve the heat resistance of the foam and provide economic advantages in relation to the ability to use less amounts of other components of the foam. For instance, when low levels of saponins are used in an AFFF formulation, the non-ionic hydrocarbon surfactant is not required. Saponins have been found to be more effective than either ethoxylated octyl phenol type or alkyl polyglycoside type surfactants for this purpose. In addition, the saponin component replaces part of the hydrocarbon surfactant component of the foam. For saponin to completely replace the hydrocarbon surfactant would require a concentration of greater than 15% (by weight) in the foam concentrate formulation. (prior to mixing with water to form the foam formulation) and this is not currently practical or desirable as saponins are relatively expensive when compared to hydrocarbon surfactants. The use of saponin also enables a reduction in the amount of fluorcarbon surfactant that needs to be used in the formulation, with resultant cost savings and improved environmental safety.
Also, when relatively low levels of saponins are added to the foam concentrate formulation--up to about 2% (by weight) but depending on the desired mixing ratio for the foam concentrate formulation in water when used for fire fighting--the heat resistance of the foam formulation produced from the concentrate formulation is surprisingly improved.
For instance, we have found that the performance of alcohol resistant AFFF (ARAFFF) formulations can be improved significantly by the use of low levels of saponin in the formulation. Alcohol resistant aqueous film forming foams (ARAFFF's) are synthetic fire fighting foam formulations with special additives to make the foam resistant to break down by polar solvents, such as alcohols or ketones, which normally break down other types of fire fighting foams. Polar solvents destroy these other types of foams by rapidly drawing water from the foam bubbles. Loss of water is rapid and results in loss of bubble strength until the bubble bursts. ARAFFF's, however, use a polymer additive to resist the drawing of water from the foam by the polar solvents. Most ARAFFF's use a high molecular weight polysaccharide, such as Xanthan gum, which slows the loss of water from the foam and forms a floating raft of thick gel which traps the bubbles on top of the polar solvent. This raft insulates the foam above it and protects subsequently applied foam from water loss.
A typical 6% ARAFFF formulation of the prior art has the following general composition:
______________________________________ COMPONENT RANGE ______________________________________ Fluorocarbon Surfactant 1.0% to 4% Amphoteric Hydrocarbon Surfactant 0% to 5.5% Anionic Hydrocarbon Surfactant 0% to 5.5% Non Ionic Hydrocarbon Surfactant 0.5% to 1% Glycol/Glycol Ether 2% to 10% High Molecular Weight Polysaccharide 0.5% to 1.3% Bactericide 0.1% to 1% Magnesium Sulphate 0% to 0.5% Urea 0% to 2% Buffer 0% to 2% Sequestrant 0% to 1% Corrosion Inhibitor 0% to 2% Water Balance ______________________________________
The polysaccharides desired to be used in ARAFFF formulations result in a foam concentrate formulation with a very high viscosity. The very high viscosity of these concentrates makes them difficult to handle and it is difficult to produce highly concentrated products which can be used with water at the most desired 3% or lower foam formulation concentrations. To date, the concentrates which are capable of being used at this level are generally based on a very expensive polysaccharide.
However, highly concentrated products remain desirable because they reduce the effective cost of the foam concentrate formulation and can reduce the total cost of foam fire fighting installations. One known method of manufacturing more concentrated ARAFFF's is to use between 1% and 20% of a polyglycoside surfactant to improve the performance in such a way that the viscosity of the final product is lower.
The most common ARAFFF's are designed for mixing with water at 6 parts foam concentrate to 94 parts water (a 6% concentrate). Products which work when mixed at 3 parts concentrate to 97 parts water (a 3% concentrate) generally have an unsatisfactorily high viscosity.
We have found that the use of a much smaller percentage of saponins in ARAFFF formulations can achieve a 3% concentrate with a satisfactory viscosity.
When relatively low levels of saponins are added to the ARAFFF concentrate formulation, (up to about 2% (by weight) but depending on the desired mixing ratio for the foam concentrate formulation in water when used for fire fighting) the ARAFFF formulation is able to use less polysaccharide while achieving the same performance.
The addition of saponins in this way to ARAFFF formulations also enables less fluorocarbon surfactant to be used.